The present invention claims the benefit of the Korean Patent Application No. P2002-13445 filed in Korea on Mar. 13, 2002, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an active matrix organic electroluminescent device including a thin film transistor.
2. Discussion of the Related Art
A cathode ray tube (CRT) has been widely used as a display screen for devices, such as televisions and computer monitors. However, a CRT has the disadvantages of being large, heavy, and requiring a high drive voltage. As a result, flat panel displays (FPDs) that are smaller, lighter, and require less power have grown in popularity. Liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescence display (ELD) devices are some of the types of FPDs that have been introduced in recent years.
An ELD device may either be an inorganic electroluminescence display device or an organic electroluminescence display (OELD) device depending upon the source material used to excite carriers in the device. OELD devices have been particularly popular because they have bright displays, low drive voltages, and can produce natural color images incorporating the entire visible light range. Additionally, OELD devices have a preferred contrast ratio because they are self-luminescent. OELD devices can easily display moving images because they have a short response time of only several microseconds. Moreover, such devices are not limited to a restricted viewing angle as other ELD devices are. OELD devices are stable at low temperatures. Furthermore, their driving circuits can be cheaply and easily fabricated because the circuits require only a low operating voltage, for example, about 5V to 15V DC (direct current). In addition, the process used to manufacture OELD devices is relatively simple.
In general, an OELD device emits light by injecting electrons from a cathode electrode and holes from an anode electrode into an emissive layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Since the mechanism by which an OELD produces light is similar to a light emitting diode (LED), the organic electroluminescence display device may also be called an organic light emitting diode.
An active matrix OELD where a plurality of pixel regions are disposed in the form of a matrix and a thin film transistor (TFT) is disposed in each pixel region is widely used in FPDs. An exemplary active matrix organic electroluminescent device is illustrated in FIG. 1.
FIG. 1 is a circuit diagram of an active matrix organic electroluminescent device according to the related art. In FIG. 1, one pixel region of an active matrix organic electroluminescent device is composed of a switching TFT 4, a driving TFT 5, a storage capacitor 6, and an organic electroluminescent (EL) diode 7. A gate electrode of the switching TFT 4 is connected to a gate line 1, the source electrode of the switching TFT 4 is connected to a data line 2, and the drain electrode of the switching TFT 4 is connected to a gate electrode of the driving TFT 5. The source electrode of the driving TFT 5 is connected to a power line 3, and the drain electrode of the driving TFT 5 is connected to an anode of the organic EL diode 7. A cathode of the organic EL diode 7 is grounded. The storage capacitor 6 is connected to the gate and source electrodes of the driving TFT 5. When a scanning signal is applied to the gate electrode of the switching TFT 4 through the gate line 1 and an image signal is applied to the drain electrode of the switching TFT 4 through the data line 2, the switching TFT 4 is turned ON. The image signal is stored in the storage capacitor 6 through the switching TFT 4. The image signal is also applied to the gate electrode of the driving TFT 5. As a result, a turn-on rate of the driving TFT 5 is determined. The current that passes through the channel of the driving TFT 5 in turn passes through the organic EL diode 7 causing the organic EL diode 7 to emit light in proportion to the current density. Since the current density is proportional to the turn-on rate of the driving TFT 5, the brightness of the light can be controlled by the image signal. The driving TFT 5 may be driven by charge stored in the storage capacitor 6 even when the switching TFT 4 is turned OFF. Accordingly, the current through the organic EL diode 7 is persistent until a next image signal is applied. As a result, light is emitted from the organic EL diode 7 until a next image signal is applied.
FIG. 2 is a schematic plan view of an active matrix organic electroluminescent device according to the related art. In FIG. 2, a gate line 21 crosses a data line 22 defining a pixel region P. A switching thin film transistor (TFT) TS is connected to the gate line 21 and the data line 22. A driving TFT TD connected to the switching TFT TS is disposed in the pixel region P. A gate electrode 41 of the driving TFT TD is connected to a drain electrode 31 of the switching TFT TS. A source electrode 42 of the driving TFT TD is connected to a power line 51 that is parallel to the data line 22. The drain electrode 43 of the driving TFT TD is connected to a pixel electrode, which is composed of a transparent conductive material. A first capacitor electrode 52 connected to the power line 51 is also disposed in the pixel region P. A second capacitor electrode 71 and 72, which is composed of polycrystalline silicon, is connected to a gate electrode 41 of the driving TFT TD. The second capacitor electrode 71 and 72 overlaps the first capacitor electrode 52 and the power line 51, thereby constituting a storage capacitor.
The organic electroluminescent device according to the related art includes a plurality of thin film transistors in one pixel region. Furthermore, since a power line is disposed in a vertical direction, the power line occupies a large portion of the pixel region. Thus, the area dedicated to the pixel electrode is reduced and the aperture ratio is accordingly reduced. As a result, the brightness of the light produced by the active matrix organic electroluminescent device is reduced.
Accordingly, the present invention is directed to an organic electroluminescent device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an active matrix organic electroluminescent device where brightness is improved due to an increase in the aperture ratio.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an active matrix organic electroluminescent device includes a substrate, a gate line on the substrate, a data line on the substrate, the data line crossing the gate line to define a pixel region, a first switching thin film transistor connected to the gate line and the data line, a first driving thin film transistor connected to the first switching thin film transistor, a power line connected to the first driving thin film transistor and parallel to the gate line, a capacitor electrode connected to the first driving thin film transistor and overlapping the power line, and a pixel electrode connected to the first driving thin film transistor and covering the pixel region.
In another aspect, an active matrix organic electroluminescent device includes a substrate, a gate line on the substrate, a data line on the substrate, the data line crossing the gate line to define a pixel region having a rectangular shape including two long sides and two short sides, a first switching thin film transistor connected to the gate line and the data line, a first driving thin film transistor connected to the first switching thin film transistor, a power line connected to the first driving thin film transistor in parallel with the two short sides of the pixel region, a capacitor electrode connected to the first driving thin film transistor and overlapping the power line, and a pixel electrode connected to the first driving thin film transistor and covering the pixel region.
In another aspect, a method of fabricating an active matrix organic electroluminescent device includes steps of forming a gate line on a substrate, forming a data line on the substrate, the data line crossing the gate line to define a pixel region, forming a first switching thin film transistor connected to the gate line and the data line, forming a first driving thin film transistor connected to the first switching thin film transistor, forming a power line connected to the first driving thin film transistor in parallel with the gate line, forming a capacitor electrode connected to the first driving thin film transistor to overlap the power line, and forming a pixel electrode connected to the first driving thin film transistor to cover the pixel region.
In another aspect, A method of fabricating an active matrix organic electroluminescent device includes steps of forming a gate line on a substrate, forming a data line on the substrate, the data line crossing the gate line to define a pixel region having a rectangular shape including two long sides and two short sides, forming a first switching thin film transistor connected to the gate line and the data line, forming a first driving thin film transistor connected to the first switching thin film transistor, forming a power line connected to the first driving thin film transistor in parallel with the two short sides of the pixel region, forming a capacitor electrode connected to the first driving thin film transistor to overlap the power line, and forming a pixel electrode connected to the first driving thin film transistor to cover the pixel region.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.